1. Field of the Invention
The present invention relates to a system for the adsorption and destruction of volatile organic compounds (VOC's) in gaseous streams which uses a single dual-function bed.
2. Related Art
Gas streams containing volatile organic compounds exhausted to the atmosphere are a source of pollution that contributes to smog and ozone at ground level. The removal of volatile organic compounds from these gas streams has been accomplished by thermal oxidation, catalytic oxidation or adsorption.
The quantity of VOC's in the gaseous streams, such as exhaust or by-product fumes from manufacturing, printing or painting, are quite low, generally in the range of 50 to 500 parts per million. Thus the gaseous stream per se is generally too dilute to serve as a fuel source. Furthermore, because of the large volume of the gaseous stream, sending the entire stream to a flare or other destruction unit is impracticable.
The conventional practice is to first capture the VOC's, e.g. by adsorption, then to desorb the VOC's in a concentrated form which can then be recovered or destroyed.
Adsorption is used to collect volatile organic compounds from a gas stream at temperatures below about 180.degree. F. In a conventional adsorption system, two or more beds of adsorbent material are used. The gas stream is passed through one bed of adsorbent material which removes VOC's. The other bed, which contains previously adsorbed VOC's, is regenerated by desorbing the VOC's in a gas stream that is heated above 180.degree. F. In some cases, the organic compounds are desorbed into a gas flow which is at a lower flow rate than the original gas stream. This results in a gas stream of a higher concentration of organic compounds. This stream can subsequently be chilled to achieve solvent recovery or incinerated by thermal or catalytic oxidation to destroy the pollutants.
Adsorbent materials include activated carbon, alumina and silica gels and molecular sieves. The amount of VOC's an adsorbent material will adsorb, depends on attributes of the particular adsorbent material such as surface area, pore size, surface composition and particle size, as well as the flow rate of the gas stream, the concentration of VOC's and the relative humidity.
In conventional adsorption systems, a hot gas is used to desorb the VOC's. In activated carbon adsorption systems, inert gas or steam must be used to avoid combustion of the carbon. The adsorption bed must be cooled after desorption to recover the adsorption capacity of the bed.
When a gas stream contaminated with VOC's is passed down flow through a bed of adsorbent material, the front of the bed collects VOC's until saturated, while the back of the bed retains adsorption capacity. In time, the entire bed will become saturated with VOC's and the concentration of VOC's exiting the bed will equal the concentration entering the bed. Some adsorbent materials will also adsorb water vapor from the gas stream, decreasing the relative humidity.
It is an advantage of the present invention that the separate catalytic or thermal oxidation units are either eliminated or reduced in size in the process of VOC removal and destruction from gaseous streams. It is another advantage of the present system that it may not require multiple adsorption units. It is a further advantage that the present single bed system uses less energy to desorb the VOC's. It is a feature of the present process that the adsorbed VOC's supply a portion of the energy for their own removal. It is a particular advantage of the present invention that the system can comprise a single dual purpose bed for adsorption/oxidation. It is a further particular feature of the present invention that present adsorption/oxidation system is operated continuously with the gaseous stream.